Fuel cells are devices that produce energy from the electrochemical reaction of a fuel and an oxidant, and they are considered attractive energy alternatives because of their high efficiency and relatively benign byproducts. Many fuel cells are being designed to use hydrogen as the fuel for the cell, but for many applications, such as use in vehicles or as portable generators, auxiliary power units (APU) or backup power supplies, it is currently impractical to procure and/or store large quantities of the needed hydrogen gas. To address this, systems are being developed to produce hydrogen on demand from more readily available or easily stored sources.
One such approach involves processing or reforming a hydrocarbon fuel into useable hydrogen. The advantages of such an approach are that a hydrocarbon fuel is generally easier to store, and significant distribution infrastructure already exists for several useful hydrocarbons, e.g. gasoline, diesel fuel, natural gas, etc. The major hydrocarbon reforming reactions typically employed require significant amounts of heat and/or elevated reaction temperatures to achieve suitable yields of hydrogen. These high temperatures and/or heat demands present challenges for system start up, such as the amount and rate of energy consumption during start up and the time delay before the fuel cell can begin producing useful energy from the produced hydrogen.
For example, the steam reforming reaction uses steam to oxidize hydrocarbons into carbon monoxide and hydrogen, typically in the presence of a catalyst. Steam reforming is strongly endothermic and is typically performed at high temperature in order to improve the kinetics and to improve equilibrium yield of hydrogen. A fuel processing system based on steam reforming has been developed by the present inventors for automotive applications (See Progress on the Development of a Microchannel Steam Reformer for Automotive Applications, G. A. Whyatt et at., 2002 AIChE Spring National Meeting). However this system has required on the order of 20 minutes to start-up from ambient temperatures to an operating temperature of around 650° C. where reasonable rates of steam reforming have been achieved. This hinders commercial practicality and fails to meet the start-up time targets established by the U.S. Department Energy for on-board fuel processors of <1 minute by 2005 and <30 seconds by 2010. Accordingly, improvements are needed, and the present disclosure describes a fuel processing system based on steam reforming that is capable of dramatically reduced start up times.
However, while the present disclosure arose from efforts to reduce the start up time for Applicants' prior automotive steam reforming fuel processing system, it will be understood that the present invention is not so limited. For example, the present invention may be applied in connection with other types of fuel processors having significant heating demands and for a variety of energy production applications. Moreover, certain components and techniques useful in the fuel processors of the present invention are also useful in other fluid processing systems. Examples include a novel mixer for efficiently mixing two fluids and an improved header design for the distribution of a gas stream to a group of microchannels in a microchannel fluid processing device.